The present invention is drawn to a method for the manufacture of an electronic package. More particularly, a leadframe is coated with an oxidation resistant layer and then that portion of the oxidation resistant layer which overlies the outer lead ends is removed generating a solderable surface.
Microelectronic circuits such as silicon semiconductor integrated circuits and hybrid microelectronic circuits require a package which both encases the circuit and provides electrical interconnection to external circuitry. A leadframe is one common means of electrical interconnection. The leadframe is formed from a strip of electrically conductive metal which is formed into a plurality of leads. The inner lead ends of the leadframe approach the integrated circuit device from one or more sides and are electrically interconnected to the device by thin bond wires. The outer lead ends of the leadframe are electrically interconnected to external circuitry such as a printed circuit board.
To protect the device from moisture and mechanical damage, the inner lead ends and the device are encapsulated. Encapsulation may be by a molding resin which surrounds both the inner leads and the integrated circuit device. Alternatively, discrete base and cover components define a cavity. When the base and cover are bonded together, the inner lead ends and integrated circuit device are encapsulated within that cavity.
Good adhesion of the inner lead ends to the molding resin is required to prevent the egress of water along the leads. Moisture can corrode the bond wires and the integrated circuit device. Additionally, the moisture accumulates inside the package. When heated, the moisture expands as steam, swelling and potentially cracking the package. This phenomenon is known as the "popcorn effect". When discrete base and cover components are utilized, the mid-portion of the leadframe is bonded to both the base and to the cover with a thermosetting epoxy or a low temperature sealing glass. Good adhesion is required to prevent the egress of moisture.
The outer lead ends of the leadframe are soldered, to external circuitry, such as a printed circuit board. These outer leads of a copper or copper alloy leadframe are solderable by removing copper oxide which formed during package assembly. Copper oxide removal is by immersion in a dilute acid. However, the copper oxide adheres poorly to the copper or copper alloy base metal and provides poor adhesion of the mid-portion of the leads to a molding resin. Better results are obtained by treating the leadframe with an adhesion promoting compound, provided, however that it is necessary that this adhesion promoting compound not detrimentally affect solderability.
Typical adhesion promoting compounds for leadframes include metals such as nickel, tin, chromium, molybdenum, aluminum and their alloys as described in U.S. Pat. No. 4,889,449 to Crane et al. Suitable polymer coatings are described in U.S. Pat. No. 5,122,858 to Mahulikar et al and include benzotriazole and ethylene vinyl acetate. The Crane et al and Mahulikar et al patents are incorporated by reference herein in their entireties.
Nickel is frequently used to coat the leads. Nickel provides an oxidation resistant coating and good adhesion to polymers. However, the elevated temperatures utilized in package assembly results in the formation of a nickel oxide layer on the outer lead ends. The nickel oxide layer is not solderable and must be removed using a strongly reducing acid such as hydrochloric acid. Hydrochloric acid can damage the molding resin and other package components and its use is not desirable.
Even if oxide free, nickel is not readily wet by lead tin solders conventionally used to mount an electronic package to a printed circuit board. Nickel coated leads are typically over coated with tin or a tin lead alloy to improve solderability. Care must be take to ensure an adequate coating of each lead, while preventing bridging between the closely spaced leads. Additionally, the electrolyte used to deposit the tin or tin lead alloy can damage the polymers used in package manufacture. As a result, the depth of package penetration in the plating bath must be accurately controlled.